Printing Paper and transporting it every day to you is costly. And back then, content was generally more expensive, because there was no elaborated online-crowd who delivered you half of your content for free. Today you have wikipedia, reddit, google, twitter, e-mail and many more which allow you to research many stuff from home. Back in the days you were forced to go around physically, talk to people face to face, visit libraries, maintain your own library. That was far more expensive than what you can do today.

At the same time, because of the high quality free knowledge we have today, you can externalize knowledge and let people research it themself if they wanne know, and get away with lower quality of writing, meaning you can do it faster and need less people to pay. Meaning you can offer a lower price, because it's neccessary to compete in a world of massivly free information.